Advancements in microelectronics and wireless communication technologies have led to the development of low cost ultra low power wireless sensors. Sensing information in large networks, i.e. power grid monitoring, large-scale home automation, etc., requires a network of low power sensors connected in a meshed topology and communicating through a wireless hopping scheme. Such a wireless sensor network consists of thousands of sensors simultaneously monitoring, transmitting, and receiving information. Although the sensors consume very little power, serving their energy needs through batteries leads to unmanageable maintenance requirements.
A typical sensor node current requirement during transmission/reception is 25-50 mA, while during the sleep mode the requirement reduces to only around 10 s of μA. Particular implementations can vary widely, but the power requirements of a simple scenario can help to illustrate the problem of maintaining power to these devices. A sample device may have a transmission/reception load requirement of 35 mA to cover a typical distance of 200 m, and a sleep requirement of 75 μA. A typical operation time for such a sensor node can be three seconds, and the sensor node can be required to transmit or receive data on average every 15 minutes. For a 9V battery with a 1200 mAh capacity, the battery would last no more that approximately 260 days. Moreover, as the distance of transmission and reception increases, the life of the battery decreases. The cost, time, and manpower involved in identifying and locating failing nodes then changing batteries for thousands of sensor nodes every few months would be prohibitive. This would discourage utilities from implementing such a technology.
Harvesting stray electromagnetic energy is a promising solution that can make wireless sensor nodes a viable technology. Since many utility assets carry current, they naturally have magnetic fields near them. The energy in the magnetic fields of these assets can be a prospective source for powering such low power sensors; however, the technology presents a number of challenges that must be overcome before the technique is practical. The energy present in the magnetic field around the utility asset fluctuates, as it is dependent on the current flowing in the asset. When the current in the asset is very small, the energy in the magnetic field is insufficient to operate the sensor. High currents, during lightning strikes or faults, may cause excessive voltage buildup on the harvester and destroy the supporting sensor electronics.
The voltage levels produced from most methods of harvesting energy are often too small to power any supporting electronic circuits. Rectification, the process of converting the harvested energy from alternating current to the direct current used in most conventional electronics, reduces the voltage level of the harvested energy by a certain amount. This amount is the threshold voltage level of the diodes used in the rectification process, and many energy harvesting methods do not result in voltage levels high enough to surpass the threshold voltage.
FIG. 1 is an illustration of a conventional power circuit 100 for energy harvesting applications. An energy harvester 110 produces an alternating current. A diode bridge rectifier 120 converts the alternating current to direct current. A gating pulse generator 150 produces a pulse that alternately opens and closes a switch 140 connecting the inductor 130 to the diode bridge rectifier 120. When the switch 140 is closed, energy builds in the magnetic field surrounded the inductor 130. When the switch 140 opens, the stored energy in the inductor 130 is released, boosting the voltage across the output. The current then flows through a diode 160 to the resistive load 180. Energy harvesters that draw energy from a magnetic field typically do not produce energy with a voltage level high enough to surpass the threshold level of the diode bridge rectifier 120 while still retaining enough voltage to operate the microcontrollers, transceivers, and other components that make up the load 180 of a wireless sensor.
Some conventional solutions can harvest enough magnetic energy to surpass the threshold level by using energy harvesters that clamp around power lines, forming a closed magnetic loop. However, the size and expense of these solutions render them non-feasible for commercial usage in networks requiring thousands of units.
Thus, there is a need for a solution that can harvest energy from a magnetic field, convert the energy from alternating current to direct current and still provide enough power to operate the supporting electronic circuitry of the wireless sensor.